7. ABSTRACT Since 2009, despite insufficient evidence, 24 states have enacted legislation that encourages supplemental breast cancer screening with advanced breast imaging to all women with dense breasts. Patient advocacy efforts that led to the legislation contend that breast density may modestly increase risk for breast cancer but reduce the accuracy of screening mammography. Nearly 31 million women have ?dense breasts?, more than 60% of whom live in states with notification legislation. Because of the potential for increased cancer detection with tests such as ultrasound and magnetic resonance imaging (MRI), their use as supplemental screening is expected to overcome some of the limitations of screening mammography. However, such use will also increase false-positives and unnecessary biopsies as ultrasound and MRI have lower specificities than mammography. Further concerns have been raised about the potential for over-treatment and overdiagnosis from supplemental screening. The critical question is whether state-level laws requiring breast density notification and encouraging use of supplemental screening tests will improve the detection of cancer, down- stage these cancers when they are found, and ultimately, increase the number of deaths averted from breast cancer. To fill this evidence gap, we will capitalize on the natural experiment formed by state-specific legislation and use interrupted time series with comparison series to analyze data from 50 states spanning 18 years (January 1, 2001 through December 31, 2018) to rigorously assess the impact of breast density reporting legislation on: 1) breast cancer imaging (including ultrasound, MRI, tomosynthesis) and diagnostic workup and patient costs using claims data from a large cohort of commercially-insured women; and 2) breast cancer detection rates using cancer registry data. Our overarching hypothesis is that density notification laws have led to a significant increase in diagnostic resource utilization without a corresponding clinically significant decrease in detection of late stage breast cancer. We have assembled a multidisciplinary research team with internationally-recognized clinicians and methodologists in health policy evaluation, particularly the use of interrupted time series and in breast density and breast cancer screening policy. This research will move the field forward through the combination of a rigorous quasi-experiment design and two unique and complementary datasets to address these critical and time-sensitive questions about benefits and harms from density notification legislation. Our research can inform the ongoing policy debates in additional states and at the federal level. Further, by examining diagnostic burden and financial cost, we will provide women and their clinicians with information essential for deciding whether to undergo supplemental screening.